Two-phase compositions of a continuous phase thermoplastic polymer and a dispersed phase elastomer produced by dynamically curing the elastomer while it is dispersed in the thermoplastic are known. See for example, Coran et al. U.S. Pat. Nos. 4,348,502, 4,130,535, 4,173,556, 4,207,404 and 4,409,365.
Fluorinated and partially fluorinated crystalline thermoplastics are high volume commercial products. Such plastics often contain large amounts of the polymerized monomer, tetrafluoroethylene.
Fluorinated and partially fluorinated amorphous elastomers are also known high volume commercial products. Such elastomers include the cured (vulcanized or crosslinked) copolymers of vinylidene fluoride with hexafluoropropylene and the like. European Patent Application No. 168,020 discloses a two-phase fluorocarbon composition in which the continuous phase is a crystalline thermoplastic fluoropolymer, and the discontinuous phase is a crosslinked fluoroelastomer in which the dispersed phase constitutes 50-90% of the total composition. The composition is prepared by dynamic curing of the fluoroelastomer phase.
The use of polytetrafluoroethylene as an additive to enhance properties of elastomers or plastics has been a long sought goal because such blends have improved properties such as tear strength, flame resistance and abrasion resistance over those of the base resin. However, because fibrils and agglomerates of polytetrafluoroethylene (PTFE) ordinarily develop when PTFE is subjected to shear forces during compounding with other resins, the resulting blends are nonuniform and may show excessive modulus and have warping problems. Furthermore, due to the fibrillation and agglomeration, blends which contain known polytetrafluoroethylene resins are difficult to prepare and process especially at high additive levels.
The incorporation of fluoropolymers such as polytetrafluoroethylene and such polymers modified with comonomer into elastomers or thermoplastics has been attempted before. High molecular weight, non-melt-processible polymers of tetrafluoroethylene, (hereinafter abbreviated TFE) including those which contain small amounts of comonomers such as hexafluoropropylene, tend to draw out into fibers, or fibrillate, when sheared. Although it is a useful property for some applications such as paste extrusion onto wire, this fibrillation causes problems when the polytetrafluoroethylene or modified polytetrafluoroethylene is to be incorporated into an elastomer or a thermoplastic. The fibrillating tetrafluoroethylene polymer forms visible agglomerates and results in undesirable increases in modulus when incorporated into elastomers. When compounded into thermoplastics, the fibrillating tetrafluoroethylene polymer causes undesirable melt swell when the polymer melt is forced through an orifice such as the die at the end of an extruder. The addition of melt-processible fluorocarbon resins such as copolymers of tetrafluoroethylene with hexafluoropropylene or perfluoro(propyl vinyl ether) or low molecular weight, irradiated PTFE, to elastomers or thermoplastics avoids the problems of fibrillation but this approach does not improve certain properties of the elastomer or thermoplastic.
It is an object of the present invention to provide a fluorinated thermoplastic elastomer that contains a tetrafluoroethylene copolymer which does not fibrillate or agglomerate when subjected to shear, and which improves extrudate quality. It is a further object of the invention to provide a composition with thermoplastic and elastomeric properties and that may further contain poly(phenylene sulfide) and an uncured perfluoroelastomer to increase the melt flow rate of the composition while improving the tensile strength and elongation properties of the composition. An advantage of the thermoplastic compositions of the invention is that flash, or defective parts can be reprocessed without difficulty. These and other objects, features, and advantages will become apparent upon the following description of the invention.